Now show sugar object choosers as file pickers in web activities

Hi,

Using the html5 file api is by far the easiest way to use files in a web activity, but it showed the gnome dialogue instead of the sugar one. This was fixed by almost copying code from browse :smile:.

Before:
![fixfileselectorbefore](https://cloud.githubusercontent.com/assets/6022042/2557300/52749e70-b6fc-11e3-9d2b-046eff148c4f.png)

After:
![image](https://cloud.githubusercontent.com/assets/6022042/2557296/14bbcf90-b6fc-11e3-9b0e-9d7e3848fe0e.png)

This is very basic, since webkit seemed to have issues with filters (`accept` attribute) :-1:.

Enjoy!